The present invention relates to a mass spectrometer used in a liquid chromatograph mass spectrometer (LC/MS) or the like with a multipole rod type ion lens.
In the mass spectrometer, there is a component called an ion lens which focuses or converges ions flying from a preceding stage, and accelerates ions circumstantially to transfer to a mass spectrograph at a subsequent stage. As the ion lens, various forms of the ion lenses have been proposed heretofore, but recently, an ion lens of a multipole rod type has been used widely. As shown in FIG. 4(a), in this ion lens (a number of rods is four in this example, but the number of the rods can be any even number, such as six or eight), voltages, in which high-frequency voltages having phases inverted with each other are superposed to the same direct current (DC) voltage, are applied to electrodes adjacent to each other (for example, electrodes designated by reference numerals 81 and 82). Ions introduced in an extending direction of a long axis (hereinafter referred to as an ion optical axis) x proceed by oscillation in a predetermined cycle due to a high frequency electric field caused by the aforementioned voltages. Thus, an effect of converging or focusing the ions is high, and a large amount of ions can be sent to the subsequent stage.
In the multipole rod type ion lens, although the focusing or convergence is satisfactory, since there is no voltage gradient in the direction of the ion optical axis x, acceleration of the ions is not carried out. Therefore, if the ion lens of this kind is used under a relatively high pressure, a kinetic energy is deprived due to collisions with the residual gas molecules, resulting in a problem that the ions passing through the lens are small.
The assignee of the invention has proposed an ion lens which uses virtual rod electrodes as shown in FIG. 4(b) (not prior art) as an ion lens which can accelerate ions while providing an ability in the convergence by the multipole rod type. In this ion lens, each rod electrode is formed of a plurality of electrode element plates 83. A voltage, in which a direct current (DC) voltage changing stepwisely toward the extending direction of the ion optical axis is superposed on the common radio frequency (RF) voltage, is applied to the plurality of electrode element plates 83 constituting a single virtual rod electrode 84. Between the virtual rod electrodes 84 adjacent to each other, phases of the high frequency components in the applied voltage are inverted, and the same DC voltage components are provided to the electrode element plates 83 existing on the same plane.
When ions generated in the ionization chamber at the preceding stage are introduced to the ion lens described above, the ions proceed while oscillating due to the electric field formed by the high frequency voltage, and the ions are converged on a rear focusing position. Also, by the predetermined direct current potential gradient in the ion optical axis direction, the kinetic energy is provided to the ions so that the ions are accelerated. Therefore, even if the ions collide with the residual gas molecules while the ions are flying, the ions proceed without extremely diverting the convergence track. Accordingly, for example, if a skimmer having a through hole communicating with the subsequent stage is disposed in the vicinity of the rear focusing position, a large number of ions can be sent to the subsequent stage via the through hole. Incidentally, a structure in which electrode element plates 85 are disposed closer to the ion optical axis x as the ions proceed as shown in FIG. 4(c) has been proposed by the assignee as well (not prior art).
Although the ion lens using the virtual rod electrodes has the excellent characteristics as described above, since the single rod electrode is separated into a plurality of the electrode element plates, a number of the components is increased naturally, resulting in the problem that the difficulty in assembly and adjustment at the time of manufacturing and using is increased.
The present invention has been made to solve the problems described above, and an object of the invention is to provide a mass spectrometer which includes a virtual rod multipole ion lens for facilitating the assembly at the time of manufacturing and the adjustment at the time of manufacturing and using.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To achieve the aforementioned object, the present invention provides a mass spectrometer using a virtual rod multipole ion lens formed of virtual rod electrodes. Each virtual rod electrode comprises a plurality of electrode element plates separated away from each other in a direction of an ion optical axis. The electrode element plate forming the virtual rod electrode has a predetermined shape at least a rim portion at an ion optical axis side. The respective electrode element plates are held and fixed at portions away from the ion optical axis by a holding unit formed of an insulator, so that the virtual rod multipole ion lens is made as a unit.
Also, apart from the virtual rod multipole ion lens unit, it is desirable to provide a terminal unit for applying predetermined voltages to the respective electrode element plates. In this case, in the virtual rod multipole ion lens unit, the electrode element plates to which the same potential is applied are connected by a stationary short line. Then, at least one of the electrode element plates which are made into groups is electrically connected to one of a socket and a plug. This connection is carried out in the respective groups. On the other hand, in the terminal unit, the other of the socket and plug is fixed at each position corresponding to one of the socket and plug connected to the electrode element plates.
Since ions proceed in the vicinity of the ion optical axis, in order to control the motions of the ions, it is sufficient to control an electric field in the vicinity of the ion optical axis adequately. In order to control the electric field in the vicinity of the ion optical axis adequately, it is enough that each electrode element plate has a predetermined shape at a rim portion thereof in the ion optical axis side. This xe2x80x9cpredetermined shapexe2x80x9d constitutes a shape theoretically determined, or a shape similar thereto and can be easily processed, and the predetermined shape is directed to a shape which can be actually used within a tolerance. More specifically, the predetermined shape constitutes a hyperbolic shape or circular arc shape.
Therefore, in the other portion, i.e. a portion away from the ion optical axis), the electrode element plate can have a favorable shape in accordance with the other condition or the like. In the mass spectrometer of the invention, the respective electrode element plates are held at the other portions thereof by the holding unit formed of the insulator, to fix the positions of the electrode element plates. Accordingly, it is possible to fix the positions securely in case of adjusting at the time of manufacturing and using the mass spectrometer. Also, since the entire ion lens including all of the electrode element plates becomes a single unit, it is very convenient to handle the lens. Incidentally, the holding unit formed of the insulator can be adequately divided as long as the holding unit as a whole is fixed in the single unit finally.
As described above, in the multipole type ion lens, high frequency voltages having reversed phases are applied to the rods adjacent to each other. Therefore, even if a number of rods is four, six or eight, two kinds of the high frequency voltages to be applied will suffice. Namely, although the direct current voltages applied to the plurality of the electrode element plates forming the single virtual rod are different, there are only two kinds of the voltages (combined voltage of high frequency voltage and direct current voltage) applied to the plurality (even number) of the electrode element plates which exist on the single plane vertical to the ion optical axis. Thus, in the virtual rod multipole ion lens unit of the mass spectrometer according to the present invention, the electrode element plates to which the same voltage (combined voltage) is applied are connected to each other by the short line (current-carrying line) in the unit. Therefore, a number of lines which should be connected to the unit can be greatly reduced. Accordingly, the connection error or failure at the time of manufacture or reassembly can be prevented, and a possibility of trouble due to contact failure can be reduced.
The short lines are made as stationary lines. This means that the positions of the short lines are fixed with respect to the entire unit, and even if the entire unit is slightly moved, the short lines do not move with respect to the entire unit.
The combined voltages applied to the ion lens are generated in a voltage applying unit disposed separately, and in order to stabilize the generated voltages, the voltage applying unit and the ion lens are adjusted to form a resonant circuit. In the liquid chromatograph mass spectrometer (LC/MS) or the like, since the ion lens used therein is gradually contaminated by the sample, it is necessary to clean the ion lens adequately. In this case, if the positions of the short lines are changed in case of removing or attaching the unit, or in case of cleaning the electrode element plates of the unit, the floatation capacity is changed, so that the cumbersome voltage adjustment has to be carried out again. By immobilizing the short lines as in the present invention, the above problem can be prevented.
The terminal unit is provided to correspond to the situation that the virtual rod multipole ion lens unit is integrally formed. Namely, by attaching the terminal unit to the virtual rod multipole ion lens unit, the electrical connection to the respective electrode element plates can be carried out at once by one action. Accordingly, the operations at the time of manufacturing and reassembling can be facilitated, and also, the change in the flotation capacity is eliminated, to thereby facilitate the adjustment of the voltage.